1. Field of the Invention
The present invention relates to a screw to be screwed into a prepared hole in a workpiece, particularly to a female-thread-formation shaving holding screw that prevents shavings generated during the female-thread forming process from falling, when the screw is screwed into a prepared hole inside of which female threads are already formed, as well as when the screw forms female threads while being screwed into a prepared hole in a relatively soft workpiece.
2. Description of the Related Art
In recent years, there is increasing need for installing electronic device parts, and screws are often used to install parts and covers. As a general procedure, as shown in FIG. 12, a prepared hole (not shown in the drawing) is provided in a workpiece 120, and a tapping screw 101 is to be screwed into the prepared hole so that the threads 110 on the shank 103 form the female threads 121 inside the prepared hole.
As for a material of which the workpiece 120 is made, materials that have relatively high recyclability are chosen in consideration of lightness in weight and environmental issues of recent years. For example, aluminum alloys and magnesium alloys are often used.
In particular, recently there are many items in which those materials are used as the following: personal computers; communication devices including mobile phones and the like; digital electric home appliances including digital cameras, digital video cameras, mini discs and the like; devices related to intelligent transport systems; and automobile parts. The workpieces made of the aforementioned materials which are often used in these products have tendency of becoming more and more lightweight and compact. Also, electric circuits used as parts of electronic devices are required to be lightweight and miniaturized; accordingly, there are developments to make screws more and more miniaturized in the present circumstances. Especially, there is an increasing demand for applying magnesium alloys to those products because of their advantageous properties such as being lightweight and energy-saving.
When such miniaturized parts need to be fixed onto a soft workpiece, tapping screws form female threads inside prepared holes while being screwed into the prepared holes to fix those parts onto the workpiece. When these female threads are formed during the screwing process, shavings such as powdered shavings due to friction and exfoliations are generated because of friction forces and deformation of the prepared holes beyond the yield point of the workpiece. These shavings may fall onto parts. Also the shavings sticking to the threads may fall when the screws are taken out of the holes. Thus, there is fear of damaging the devices or sometimes even causing a fire when the shavings fall onto an electric circuit of a printed circuit board and develop a short circuit.
When the workpiece is made of a soft material, during the process of forming female threads with tapping screws, normal tapping screws are likely to suffer from friction heat due to their designs in which the width of each root in the threads is small. Consequently, there may be a situation where a tapping screw is burnt onto the workpiece and does not get screwed in all the way, and we may have what is called a stick-out, which means that the bearing surface of a screw head is not in contact with the workpiece. Particularly, since magnesium alloys are soft and brittle, fine particles are likely to be generated, and they cause adhesion in the space between the tapping screw and the threads during the screwing process; therefore, it is not possible to achieve self-tapping action, which means that the screw forms female threads while being screwed into the workpiece, and it is necessary to form female threads in advance inside the prepared hole.
In order to fix some parts onto a workpiece, however, even when a normal screw is screwed into the workpiece after female threads are formed in advance inside the prepared hole in the workpiece, there is a possibility that a few shavings may be generated between the screw and the workpiece and fall, and this could still be one of the main causes of troubles in electronic devices. Moreover, since it is necessary to perform an assembly step of forming female threads in advance, the manufacturing cost increases, and a problem may arise that there is a limit to the possibility of supplying inexpensive products.